Free Electrostatics MCQs with Answers
25 Electrostatics MCQs from Physics, each with the correct answer and a written explanation of why it is correct. Free and unlimited, with no account needed.
25 questions · page 3 of 3
21. The electric field at a point due to two equal and opposite charges is 100 N/C. If the magnitude of each charge is doubled then the electric field at that point becomes:
- A. 50 N/C
- B. 100 N/C
- C. 200 N/C
- D. 400 N/C
Explanation: This is the option marked correct in the official UHS answer key. Field intensity is directly proportional to the charge producing it, so doubling both charges doubles each contribution and doubles the resultant, which by that reasoning gives 200 N/C. The key's figure of 400 corresponds to treating the effect as multiplying by four, so this is a question where the official answer and the standard proportional argument diverge.
Correct answer: 400 N/C22. If a plastic sheet of relative permittivity 2.5 is inserted between two point charges placed in vacuum, then the electrostatic force between them
- A. Increases by a factor of 2.5
- B. Decreases by a factor of 2.5
- C. Increases by a factor of 5
- D. Decreases by a factor of 5
Explanation: Relative permittivity appears in the denominator of Coulomb's law, so inserting a medium of relative permittivity 2.5 reduces the force to 1 over 2.5 of its vacuum value. Water, with a relative permittivity of about 80, weakens ionic attraction eightyfold, which is why ionic compounds dissociate so readily in it. Air is close enough to 1 that the vacuum formula is used for it.
Correct answer: Decreases by a factor of 2.523. If the surface charge density of an infinite sheet increases by 25%, the electric field intensity:
- A. Increases by 25%
- B. Increases by 50%
- C. Decreases by 25%
- D. Remains the same
Explanation: The field of an infinite charged sheet is the surface charge density divided by twice the permittivity, so it is directly proportional to that density and rises by the same 25 per cent. Notably the field does not depend on distance from the sheet at all, which is what makes the infinite sheet such a useful idealisation. A point charge behaves quite differently, falling off as the inverse square.
Correct answer: Increases by 25%24. For two equal positive charges, the electric field is weakest:
- A. Midway between them
- B. Along the perpendicular bisector
- C. Close to either charge
- D. At infinity
Explanation: At the midpoint the two fields are equal in magnitude and opposite in direction, so they cancel exactly and the resultant is zero, which is the weakest the field can be anywhere at finite distance. Close to either charge the field is strongest. The field also tends to zero at infinity, but the question is asking for the null point produced by the pair.
Correct answer: Midway between them25. If the distance between two charges is halved and magnitude of charges are also doubled, then the force between these charges becomes:
- A. Two times
- B. Four times
- C. Eight times
- D. Sixteen times
Explanation: Doubling both charges multiplies the numerator of Coulomb's law by four, and halving the separation divides the denominator by four since the distance is squared, so the two effects combine to give sixteen times the original force. Handling the two changes separately and then multiplying is the safe method. Forgetting that the distance is squared gives the eight times answer.
Correct answer: Sixteen times